1. Field of the Invention
The present invention relates to data communications, and more particularly, to detection of transmission paths compatible with digital modulation.
2. Description of the Related Art
Much of the public switched telecommunications network (PSTN) is implemented using digital data transport. Nonetheless, significant portions of the PSTN are still based on analog technology. For example, the “local loop” portion of PSTN that connects a telephone subscriber to a central office (CO) is typically an analog loop. Additionally, analog portions may exist at other points along a communications path, e.g., as an analog channel in an otherwise digital circuit.
The current generation of 56 Kbps modems (e.g., those based on K56flex™ or x2™ technology or conforming to ITU-T Recommendation V.90) no longer assume that both ends of a communications path may be analog and suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs). Instead, such modems assume that there is only one (1) analog portion in a downstream transmission path from a digitally connected server modem to a client modem connected to an analog local loop. This assumption is reasonable in areas where most Internet Service Providers (ISPs) and business customers are digitally connected to the network and allows data signaling rates of up to 56 Kbps in the downstream transmission path. K56flex is a trademark of Lucent Technologies Inc. and x2 is a trademark of 3Com Corporation.
Although a variety of similar designs are available, modems conforming to the ITU-T Recommendation V.90 are illustrative. See generally, ITU-T Recommendation V.90, A Digital Modem and Analogue Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signalling Rates of up to 56 000 Bit/S Downstream and up to 33 600 Bit/S Upstream (09/98), the entirety of which in incorporated by reference herein. Recommendation V.90 defines a method for signaling between a modem connected to an analog loop (the analog modem) and a modem connected to the digital trunk (the digital modem). Modems in accordance with Recommendation V.90 take advantage of this particular arrangement to increase the data signaling rate from the digital modem towards the analog modem. The quantization noise from a mu-law or A-law PCM converter typically limits the signal-to-noise ratio (SNR) to about 38 dB. However, the detrimental effect of quantization noise can be avoided in certain cases. If there are no analog-to-digital conversions in the downstream path from the digital V.90 modem to the analog modem, the PCM codes from the digital modem are converted to discrete analog voltage levels in the local CO and are sent to the analog modem via the analog local loop. The analog modem's receiver then reconstructs the discrete network PCM codes from the analog signals received. Using current techniques, 56 Kbps signaling rates can be achieved.
There is no specific means provided in Recommendation V.90 by which the analog client modem is to decide whether PCM signaling can be supported by the downstream channel. Rather, the client modem must make an inference about the condition of the channel during the training process. For example, one technique apparently used by some modem manufacturers is to use the L1L2 signal which is transmitted as part of probing and ranging in phase 2 of startup procedures specified in Section 9.2 of Recommendation V.90 to estimate the high-end frequency response of the channel. A channel over which PCM operation is possible will typically exhibit less roll-off at frequencies near the upper edge of the voice band (0–4000 Hz) than will channels that cannot support PCM. There is no assurance, however, that good high-frequency response will correlate with good PCM operation, and poor high-frequency response by itself does not prelude PCM operation. Therefore, such methods assume a relationship between high-frequency response and PCM operation that is not valid in many practical cases. If the client modem fails accurately to assess the channel, then subsequent data exchange either will fail or will occur at less than optimal speeds.